The present invention relates generally to the design and construction of microstrip antennas. More particularly, the invention relates to microstrip antennas having a plurality of interconnected segments which are disposed on successive layers of a multilayer substrate.
Typically, half wavelength patch, microstrip, and stripline antennas (hereinafter referred to collectively as "microstrip" antennas) are generally required to have a length: ##EQU1## where .function. is the operating frequency of the antenna, c is the speed of light in a vacuum, and .epsilon..sub.R is the relative dielectric constant of the substrate.
FIG. 1A shows a perspective view of a typical prior art half wavelength microstrip antenna 100. FIG. 1B shows a side view of the prior art antenna 100. According to prior art designs, the microstrip antenna 100 can have a variety of geometries, such as rectangular, circular, or pentagonal. Such antennas are typically constructed by forming an electrical conductor 102 on top of an electrically insulating substrate 104. The method of electrical connection to the conductor 102 can vary. By way of example, antenna 100 is shown adapted for connection through a probe feed 106. Alternatively, antenna 100 can include a microstrip connection or a capacitively coupled connection.
During operation, the conductor 102 radiates in response to receiving a signal having a wavelength, .lambda., equal to twice the length of the conductor 102. That is: ##EQU2## where .epsilon..sub.Effective is the; effective relative dielectric constant of the antenna. As is well known, the value of .epsilon..sub.Effective is a function of the geometry of the conductor 102, in addition to .epsilon..sub.R. Typically .epsilon..sub.Effective approaches .epsilon..sub.R as W/h becomes large, where W is the width of the conductor and h is the thickness of the substrate 104.
FIG. 2 shows an equivalent electrical circuit for the half wavelength antenna of FIGS. 1A and 1B. As shown, the resonant antenna 100 can be viewed as a half wavelength transmission line, with capacitors C.sub.edge1 and C.sub.edge2, corresponding to the fringing fields, in combination with the relatively high resistances R.sub.edge1 and R.sub.edge2 corresponding to the radiation resistance of the radiating edges 102a and 102b of the conductor 102.
The prior an antenna 100 shown in FIGS. 1A and 1B suffers from the drawback: that its actual length varies inversely with the frequency at which the antenna 100 operates. Consequently, antennas designed for operation below a few Gigahertz or so, when constructed with substrates made from conventional ceramics or other conventional materials, are far too large and heavy for many applications. Additionally, where those large and heavy antennas can be used, the size results in excessive manufacturing and materials costs.
Some prior art systems reduce the antenna size by using materials with higher dielectric constants. However, many of these materials have undesirable properties, not present in lower dielectric constant materials. Such properties of concern include: the temperature coefficient of expansion; the temperature coefficient of dielectric constant; the dissipation factor (Q); the thermal conductivity; the environmental stability; and the durability. Also, microstrip antennas constructed on high dielectric constant substrates often excite undesirable modes, such as for example, surface waves which detract from the radiated power in the desired mode of operation. Further, higher dielectric substrate materials are generally more expensive than conventional substrate materials.
Consequently, an object of the present invention is to provide a microstrip antenna having a reduced size.
Another object of the present invention is to provide a microstrip antenna having a reduced size, and constructed from conventional materials.
A further object of the present invention is to provide a small, lightweight microstrip antenna for operation in a range of frequencies below a few Gigahertz.
Other general and specific objects will in part be obvious and will in part appear hereinafter.